1. Field of the Invention
The present invention relates to a method of detecting parameters, particularly, the thickness of the liquid crystal layer and the angle of twist of liquid crystal molecule orientation, of a liquid crystal cell for use in a liquid crystal display and the like.
2. Description of the Prior Art
In liquid crystal cells, all of the liquid crystal molecules are oriented parallel to the plane of the two substrates and the direction of orientation is gradually twisted between the two substrates. Examples of such liquid crystal cells include a twisted nematic liquid crystal cell (hereinafter referred to as "TN cell") in which the direction of orientation is twisted exactly 90 degrees and a super twisted nematic liquid crystal cell (hereinafter referred to as "STN cell") in which the direction of orientation is twisted between 180 degrees and 270 degrees. These liquid crystal cells are widely used in various liquid crystal displays such as displays for word processors and personal computers.
In such TN and STN cells, the display quality greatly depends on the distance between the two substrates, i.e. the thickness of the liquid crystal layer, and the angle of twist of the liquid crystal molecule orientation.
The anchoring energy (i.e., the orientation restriction force in the direction of azimuth on the liquid crystal cell substrate plane), which is one of the important parameters of liquid crystal cells, can be obtained from a variation of angle of twist of the liquid crystal molecule orientation between the two substrates.
Therefore, in the manufacture of liquid crystal cells, it is extremely important to quickly and accurately detect the thickness of the liquid crystal layer and the angle of twist of the liquid crystal molecule orientation.
Conventionally known methods for determining the thickness of a liquid crystal layer include (1) a method in which capacitance and optical phase difference of the liquid crystal cell are measured to thereby calculate the thickness of the liquid crystal layer, (2) a method in which light interference and capacitance are measured in an empty cell state, i.e., where no liquid crystal has been sealed in yet, to thereby calculate the thickness of the liquid crystal layer, and (3) a method in which the distance between the two substrates is measured by use of a three-dimensional shape measuring method to thereby calculate the thickness of the liquid crystal layer.
Known methods for determining the angle of twist of liquid crystal molecule orientation include a method in which (1) a liquid crystal cell is interposed between two polarizing plates, (2) the optical path difference is adjusted by a photoelastic modulation element and (3) curve fitting of the transmitted light intensity is performed, thereby determining the angle of twist of liquid crystal molecule orientation.
Moreover, a method is known in which (1) a liquid crystal cell is interposed between two polarizing plates, (2) one or both of the liquid crystal cells and the polarizing plates are rotated to thereby obtain an angle in which the transmitted light intensity is at a maximum or minimum, and (3) the thickness of the liquid crystal layer and the angle of twist of liquid crystal molecule orientation are obtained by use of a Jones' matrix display.
These methods are described in "J. Appl. Phys." (vol. 69, p.1304), "Jpn. J. Appl. Phys." (vol. 33, pp. L434-L436), "Jpn. J. Appl. Phys." (vol. 33, pp. L1242-L1244), "Jpn. J. Appl. Phys." (vol. 35, pp. 4434-4437) and "Manuscripts of Lectures at the 22nd Japanese Liquid Crystal Conference" (pp. 139-140), etc.
However, according to the conventional methods, it is difficult to accurately determine the thickness of the liquid crystal layer because the capacitance and the optical difference of the liquid crystal cell are measured. In addition, the apparatus is complicated and expensive since an optical path difference modulation element such as a photoelastic modulation element and a phase compensation element is necessary. Further, the apparatus is complicated and expensive and errors are apt to be caused since curve fitting of the transmitted light intensity is performed or the polarizing plates and the liquid crystal cell are rotated to measure an angle where the transmitted light intensity is maximum or minimum.
To overcome the above-mentioned problems, the inventors performed various studies and found that the thickness of the liquid crystal layer and the angle of liquid crystal molecule orientation can be obtained based upon Stokes parameters and that the Stokes parameters can be accurately measured in a short time with a simple apparatus.